The present invention is related to a transmission system comprising a transmitter with a signal encoder having an input for a signal to be encoded, said signal encoder comprises a codebook entry selector for selecting a codebook entry for obtaining a synthetic signal giving a best approximation of a signal representative of the input signal, the codebook entry comprises a plurality of samples that can assume more than two values, said codebook entry being identified with a sequence of symbols, the transmitter being arranged for transmitting the sequence of symbols to a receiver, the receiver comprises a decoder with a codebook for deriving the codebook entry from the received sequence of symbols.
A prior art transmission system is known from the conference paper xe2x80x9cAn algorithm for assigning binary indices to the code vectors of a multi-dimensional quantizerxe2x80x9d by J. De Marca and N. Jayant published in the proceedings of the IEEE International Conference on Communications ""87(ICC-87), Volume 2, pp. 1128-1132.
Such transmission systems are e.g. used in applications in which speech or video signals have to be transmitted over a transmission medium with a limited transmission capacity or have to be stored on storage media with a limited storage capacity. Examples of such applications are the transmission of speech signals over the Internet, the transmission of speech signals from a mobile phone to a base station and vice versa and storage of speech signals on a CD-ROM, in a solid state memory or on a hard disk drive.
In a transmission system according to the preamble, the signal to be encoded is compared with a plurality of synthetic signal segments. Each of the synthetic signal segments is derived from one of the codebook entries. The synthetic signal segments can e.g. be obtained by filtering the sequence of samples contained in the codebook entry by means of a synthesis filter. The codebook entry corresponding to the synthetic signal segment which best matches the input signal is encoded and transmitted to the receiver.
An alternative possibility is to derive a residual signal from the input signal by means of an analysis filter and to compare the residual signal with each of the codebook entries. The codebook entry best matching the residual signal is encoded and transmitted to the receiver.
It is also conceivable that the input signal is directly compared with the codebook entries and that the best matching codebook entry is encoded and transmitted.
In the receiver, the received code associated with the codebook entry is decoded and a replica of the input signal is reconstructed. This can be done by applying the plurality of samples to a synthesis filter which has a similar transfer function as the synthesis filter used in the encoder. If an analysis filter is used in the encoder, a synthesis filter is used which has a transfer function which is the inverse of the transfer function of the analysis filter.
If no analysis or synthesis filter is used in the encoder, the reconstructed signal is directly derived from the decoded codebook entry.
It can happen that due to transmission impairments, the encoded codebook entry is received in error. Consequently, in the receiver a codebook entry different from the codebook entry selected in the encoder will be used for reconstructing the input signal. Using the wrong codebook entry for reconstructing the input signal will in general result in an audible/visible error in the reconstructed signal.
In the transmission system according to the above mentioned conference paper it is tried to minimize the effect of transmission errors by assigning to similar codebook entries similar sequences of symbols in such a way that if a transmission error occurs in one of the symbols, the codebook entry corresponding to said erroneously received sequence of symbols differs only slightly from the codebook entry corresponding to the originally transmitted sequence of symbols. In this way it is obtained that the perceptual effect of a transmission error is substantially reduced.
The object of the present invention it to provide a transmission system in which the perceptual effect of transmission errors is even more reduced than in the prior art system.
To achieve said object the present invention is characterized in that the codebook entries corresponding to sequences of symbols differing in one particular symbol value, differ in one single sample value. This particular symbol value can be the least significant symbol, but it is also possible that it is a symbol at a different position in the sequence of symbols.
For the purpose of designing the assignment of sequences of symbols to codebook entries in the prior art system, it is assumed that every symbol in the sequence of symbols can be in error. This assumption results in a non-optimum assignment of codebook entries to sequences of symbols when it is taken into account that the possibility of a transmission error often differs for several symbols. It is possible that an error correcting code is used for a part of the sequence of symbols. It is also possible that hierarchical modulation is used resulting in different error probabilities. By restricting the number of symbols which can be in error, it becomes possible to reduce the difference between the codebook entries.
By making codebook entries differing in one single sample to correspond to sequences of symbols differing in one particular symbol value (mostly the most vulnerable one) a near optimum codebook is obtained.
An embodiment of the present invention is characterized in that the difference between said sample values of codebook entries corresponding to sequences of symbols differing in one particular symbol value, is equal to a smallest quantization step of said sample value.
By choosing the difference between the sample values corresponding to xe2x80x9cneighboringxe2x80x9d sequences of symbols equal to the smallest quantization step, an optimum codebook with respect to the perceptual effect of a single transmission error is obtained.
A further embodiment of the invention is characterized in that the number of possible sample values is odd. It is found that in the case of an odd number of possible values it becomes possible to calculate the mapping between sequences of symbols and the corresponding plurality of samples and its inverse with the same algorithm. This results in a reduced amount of resources required to implement a combination of encoder and decoder, because the resources for performing the codebook related calculation can be shared.
If the combination of encoder and decoder is realized by a program running on a programmable processor, the amount of memory to hold the program is reduced. If the combination of encoder and decoder is realized in hardware, the amount of chip area will be reduced because the part for determining the sequence of symbols from the plurality of samples can also be used for determining the plurality of samples from the sequence of symbols.
A still further embodiment of the present invention is characterized in that a numerical value associated with a first codebook entry is equal to the numerical value of the sequence of symbols of a second codebook entry, and in that the numerical value associated with the second codebook entry is equal to the numerical value of the sequence of symbols associated with the first codebook entry.
According to this aspect of the invention, it becomes possible to determine the index of a given codebook entry by first using said given codebook entry as index to determine a second codebook entry and secondly by using the second codebook entry as index to determine a codebook entry which represents the index of the given codebook entry.